Several standards are available for encoding and decoding video to reduce the bandwidth of the video such as H.261, H.262, H.263, MPEG-1 and MPEG-2. These standards typically utilize both temporal and spatial compression. A to be encoded video picture (field or frame) is divided into non-overlapping macroblocks, where each macroblock is a two dimensional array of luminance blocks and each chrominance block overlapping thereon. Each luminance or chrominance block, in turn, is an 8.times.8 array of (luminance or chrominance) pixel data. In temporal encoding, one or more reference pictures are designated for each to be encoded video picture. For each macroblock of the to be temporally encoded video picture, a search is carried out, e.g., using a block matching technique, to identify a prediction macroblock in a reference picture which best matches the macroblock of the to be encoded picture. This is referred to as motion estimation. The prediction macroblock is identified by a motion vector which indicates a spatial (pixel row and column) and temporal (picture of the sequence) offset from the macroblock of the to be encoded picture to the prediction macroblock. A prediction error macroblock is then formed by subtracting the macroblock of the to be temporally encoded picture from the prediction macroblock. The individual blocks of the prediction error macroblock are then spatially encoded.
Spatial encoding involves the steps of discrete cosine transforming (orthogonally transforming) a block, quantizing the transformed block, and entropy encoding the quantized block (e.g., run-level encoding the quantized block and variable length encoding the run-level encoded block). The encoded data thus produced is formatted into a bitstream including other header, flag and parameter data.
According to the MPEG-2 standard, it is not strictly necessary to encode each macroblock. Rather, if a prediction error macroblock has a zero spatial offset motion vector, and only zero valued quantized block data, no data need be provided in the bitstream for the macroblock. Instead, indicator information is placed in the bitstream to indicate that a macroblock has been skipped.
It is an object of the present invention to provide an improved determination process for determining whether or not to skip a macroblock.